1. Field of the Invention
This invention relates to electrical devices comprising conductive polymer compositions.
2. Summary of the Prior Art
Conductive polymer compositions comprising a conductive carbon black dispersed in a polymer are well known. Over recent years, there has been particular interest in such compositions which exhibit positive temperature (PTC) characteristics, i.e. which show a very rapid increase in resistivity over a particular temperature range. Reference may be made for example to U.S. Pat. Nos. 2,978,665; 3,243,753; 3,351,882; 3,412,358; 3,413,442; 3,591,526; 3,673,121; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,914,363, and 4,017,715; 4,177,376, 4,177,446, 4,246,468, 4,388,607, 4,421,582, 4,426,339 and 4,534,889. British Patent No. 1,409,695; Brit. J. Appl. Phys. Series 2, 2 569-576 (1969, Carley Read and Stow); Kautschuk und Gummi II WT, 138-148 (1958, de Meij); Polymer Engineering and Science, Nov. 13, 1973, No. 6, 462-468 (J. Meyer); U.S. Patent Office Defensive Publication No. T 905,001; and commonly assigned U.S. patent application Ser. No. 601,639. The disclosure of each of the above patents and applications is incorporated herein by reference.
PTC compositions are useful, inter alia, in electrical devices comprising a PTC element in combination with another resistive element whose resistance remains relatively constant at least up to the temperature range in which the PTC element shows a very rapid increase in resistance, such other element being referred to be as a constant wattage (CW) [or relatively constant wattage (RCW)] element. It is to be noted that the resistance of a CW element need only be relatively constant in the temperature range of normal operation; thus it can decrease, remain constant, or increase slowly in this range, and can exhibit PTC characteristics above normal operating temperatures of the device. Such devices are described for example in U.S. Pat. No. 4,017,715 and German Offenlegungschrift Nos. 2,543,314.1 and 2,903,442.2. In order to obtain the best results from such devices, it is necessary that the resistivities of the PTC and CW elements should be correlated throughout the temperature range of operation and in many cases that the resistivity/temperature characteristics of the elements and the contact resistance between the elements (whether bonded directly to each other, as is generally preferred, or through a layer of a conductive adhesive) should not change excessively on storage or in use, eg. due to temperature variations which take place during operation of the device. The CW compositions hitherto available are not fully satisfactory in these respects. For example, it is well known that certain conductive polymer compositions comprising an elastomer and a carbon black exhibit CW behavior, but unfortunately the resistivity of such compositions is excessively dependent on their thermal history.